THE SCIENCE BEHIND THE QUAKE:Stresses along what is known as a 'line of subduction' could now put Tokyo at risk
THE CATASTROPHIC earthquake off Japan will certainly rank in the top 10 most powerful quakes ever and possibly in the top five, according to an Irish expert.
The event has also increased the risk that Tokyo could experience a similarly powerful earthquake, said Prof John McCloskey a geophysicist at the University of Ulster. He was speaking yesterday as tsunamis continued to spread across the Pacific Ocean to threaten coastlines from China to California.
Tsunami waves reaching shore at up to 10m high have been recorded, but the heights of waves could vary depending on distance from the quake and local seabed conditions, he said.
There have also been many aftershocks and any of these could create fresh tsunamis.
The event is currently ranked as an 8.9-magnitude quake but Prof McCloskey believes that it will be upgraded to 9 or even higher as more data comes in about what actually happened on the sea floor some 350km due east of the Japanese city Sendai.
“We already know a lot about it and more information is coming in all the time,” Prof McCloskey said. “The earthquake happened where the Pacific plate is being forced under Japan in an interface called a subduction zone.”
He believes that yesterday’s extremely powerful event was triggered by a smaller quake that took place earlier this week. “Two days ago a magnitude 7.2 earthquake occurred on the subduction zone 30-35km below the sea surface. This earthquake deformed the rocks in the earth’s crust.”
This greatly increased the build-up of stress where the Pacific seafloor is being forced down under Japan. This stress eases only when one body of rock slips against another, in the process releasing staggering amounts of energy.
“We are absolutely sure this event today [off Sendai] was triggered by the earlier quake,” Prof McCloskey said. “The event was a truly globally massive earthquake. It is definitely going to be one of the top 10 biggest earthquakes of all time and could be in the top five.”
The stresses occur because the earth’s outer shell or crust is not uniform. It is broken up into plates, some of them bearing land masses and others making up the seabed, explained Tom Blake of the Dublin Institute for Advanced Studies.
These jostle and crush against one another in different ways, some sliding sideways against one another and others pushing against one another head on.
Colliding Indian Ocean and Asian plates have built up the Himalyan mountains. California lies on the edge of sliding plates and so is vulnerable to earthquakes.
This earthquake, however, was caused by subduction where the Pacific plate slides under the Eurasian plate. Japan lies on the eastern edge of the Eurasian plate and so is right on top of a region where earthquakes are common.
Subduction quakes shake the earth but also trigger tsunamis, Prof McCloskey said. The Pacific plate is forced downwards but the top plate is also dragged down because of friction. Stresses begin to build and finally reach breaking point in what is known as a megathrust earthquake.
“The overriding plate basically bounces back up after a period of 100 or more years, when the elastic forces get too strong for the frictional forces to hold,” he said. “All the stress is released in a few hundred seconds.”
When it broke free a massive area on the edge of the Eurasian plate sprang up, displacing an equal volume of water. This bulge of water was pushed towards the surface.
“That bulge drops back into the ocean, causing what is like a big splash and the waves move away from it,” Prof McCloskey said. These tsunami waves are fairly small when in deep ocean, less than a metre high such that even small boats can easily manage them.
The massive amount of water thrown out from the quake rushes across the ocean but only manifests its overwhelming power when it rides up on shore to produce high waves that can travel inland for kilometres.
Prof McCloskey has already completed calculations on just how big a megathrust displacement occurred. He estimates that a piece of the Eurasian plate at least 500km long and 200km wide pushed upwards between 4-5m.
While the quake’s shaking of the earth would have killed some when buildings broke up, the effects of the tsunami as it spreads all around the Pacific Ocean will cause substantially more deaths. Hundreds are already feared to have died in Japan as the waves swept on shore.
People caught in a tsunami of even one metre would almost certainly not survive, and some of the waves that reached land were up to 10m, Prof McCloskey said.
The risk of an even greater catastrophe has been increased because of the Sendai earthquake, Prof McCloskey said. He is in no doubt that the smaller quake earlier this week caused stresses that in turn broke free in yesterday’s event.
The Sendai quake will in turn cause stresses to build further south along the line of subduction. Tokyo lies further down this line, he pointed out.
The Pacific plate is subducting all along a line southward from Alaska to Japan and on as far as the Philippines, he said. “You have a long, thin strip where subduction happens.”
The release of stress off Sendai will pass this down along the line. “This quake will have increased stress in the area south of Tokyo,” he said. Tokyo is about 100km further down along the fault line and may be in greater jeopardy as a result. The Sendai quake “certainly made this more likely”, Prof McCloskey said.
There would be loss of life in the event of a strong quake but damage to such an important international financial centre would have an economic impact running into the trillions of dollars, he believes.
“A big earthquake in Tokyo would have a world impact. This [Sendai] earthquake will have cost less than 1 per cent of Japan’s GDP. The economic impact of a Tokyo earthquake is significantly bigger.”
